Although the use of dual pole solenoids appears to dominate in most solenoid applications, single pole solenoids still remain preferred in some applications. In most dual pole solenoid designs, an armature is spaced at an air gap distance away from a stator having a coil embedded therein. When the coil is energized, magnetic flux is generated around the coil, and flux lines pass through the stator, the armature and back to the stator. The resulting flux path produces a pair of magnetic north and south poles between the stator and armature on each side of the air gap. The flux between these poles is parallel to the armature motion. These opposite poles produce a force on the armature that tend to move it in the direction of the stator and coil to accomplish some task, such as close a valve, etc. In the case of a single pole solenoid, a magnetic flux path is created around the coil. In a typical single pole solenoid, the magnetic flux path also encircles the coil and passes through the stator, the armature, and back to the stator. The resulting flux path also produces a pair of magnetic north and south between the stator and the armature. In the single pole configuration, the flux between the poles is parallel to armature motion for one set of poles and perpendicular to armature motion for the other set of poles. Only one set of poles is producing magnetic force for armature motion. In both single and dual pole designs, the armature generally moves toward the stator to reduce the size of the air gap their between.
In many single pole solenoid designs, the armature must also have a radial sliding gap with respect to another electro magnetic component that is present to complete the magnetic circuitry around the coil. Due primarily to manufacturing considerations, this extra magnetic piece is often not included as a portion of the stator, but is generally in contact with the stator, stationary and positioned to complete the magnetic circuit around the coil. Depending upon the configuration of the single pole solenoid, this additional magnetic component is sometimes referred to as a magnetic flux ring. When the coil is energized, the magnetic flux lines encircle the coil but pass sequentially through the stator, the armature, the magnetic flux ring and back to the stator, or vice versa. Since the magnetic flux ring is stationary but the armature moves, there must be some sliding air gaps between these two components. However, those skilled in the art will appreciate that this sliding gap is preferably as small as possible in order to produce the highest possible forces on the armature. When this sliding air gap becomes so small that the armature touches the magnetic flux ring, a high magnetic force is produced but the armature is unable to move. When the sliding gap becomes too large, the magnetic flux can sometimes tend to seek out a lower reluctance path than spanning the sliding gap such that the solenoid can begin to perform like a poorly configured dual pole solenoid.
In general, for a given space and small initial air gap, a dual pole solenoid can almost always be designed that will produce higher forces than that of a single pole solenoid for similar sized initial and final air gaps. This fact usually results in a designer choosing a dual pole solenoid design over a corresponding single pole design.
In some applications, such as in fuel injectors where a single solenoid is moving two different valve members, it is desirable that the solenoid have the ability to stop at an intermediate position. In many instances, it is desirable that the armature have the ability to move from its de-energized position to the intermediate position as quickly as possible; however, it is also often desirable that the solenoid have the ability to stop the armature at the intermediate position without substantial overshooting or significant oscillations about that intermediate position. In many instances, this intermediate position is accomplished by balancing the solenoid force against a compressed spring having a predetermined pre-load. When, as in the case of many fuel injectors, where total armature movement is only on the order of tens of microns, the ability to produce multiple fuel injectors that perform substantially identical when the various components that make up the assembly must inherently have some dimensional tolerancing, is extremely difficult.
The present invention is directed to overcoming these and other problems associated with producing large quantities of solenoid assemblies that perform reliably, uniformly while remaining realistically manufacturable.